Heat treatment of niobium alloys



United States Patent Ofiice 3,366,513 Patented Jan. 30, 1968 3,366,513 HEAT TREATMENT OF NIOBIUM ALLOYS Anthony Clifford Barber, Birmingham, and Rodney Trenchard Bryant, Sutton Coldfield, England, assignors to Imperial Metal Industries (Kynoch) Limited, London, England, a corporation of Great Britain No Drawing. Filed Feb. 15, 1965, Ser. No. 432,891 Claims priority, application Great Britain, Feb. 20, 1964, 7,161/64 7 Claims. (Cl. 148-2) ABSTRACT OF THE DISCLOSURE A heat treatment for improving the ductility of niobium alloys containing -25 wt. percent tungsten, 0-10 wt. percent molybdenum and a dispersion of hafnium carbide in which the alloy, having a brittle uniformly distributed coarse plate-like carbide structure formed above 1600" C., is heated between 1 000 and 1250 C. to transform the coarse carbide into a fine structure possessing good ductility.

This invention relates to the heat-treatment of niobiumbase alloys containing a hafnium-bearing carbide. Examples of such alloys are described in our co-pending application for Letters Patent No. 335,029.

The said alloys contain 0-25 wt. percent tungsten, 0-10 wt. percent molybdenum and up to 8 wt. percent hafnium with sufficient carbon to form a dispersion of carbide. The carbide contains hafnium and carbon but may not be of a simple formula corresponding to hafnium carbide since it may also contain niobium or other elements forming a complex mixed carbide.

We have found that the ductility of the alloys is dependent upon the form and distribution of the carbide and have discovered a heat-treatment which modifies the carbide so as to produce increased ductility whilst retaining high strength.

The present invention provides a method of heattreating niobium-base alloys containing a dispersion of hafnium-bearing carbide comprising first heating the alloy at a temperature above 1550 C. and secondly heating the alloy at a temperature between 1000 C. and 1350 C. The first heat-treatment is preferably between 1600 C. and 1900 C. and the second between 1100 C. and 1250 C.

The heattreatment may be applied to the alloys in two ways. Firstly, the alloys, for example, in the form of forgings, may be treated at the temperatures given above as separate reheating steps, and secondly, a melting operation or a fusion welding operation may be utilised as the first heating step, since in such operations, as in the first, separate reheating step, a coarse form of carbide is produced.

The times of heat-treatment are not critical and will depend upon the temperature. Suflicient time should be allowed for the structure of the alloy to approach equilibrium. The first heat-treatment at elevated temperature should preferably not exceed two hours in order to avoid excessive grain growth and in practice a range of times between minutes and 2 hours is suitable. For the second heat-treatment, at the lower end of the temperature range about 3 hours are required, but the upper limit is indeterminate since a stable structure is obtained. Towards the upper end of the temperature range (e.g. 1250 C.) for the second heat-treatment about /z-hour is sufficient and times in excess of 5 hours should be avoided since the strength is decreased. The temperature range for the second heat-treatment is, therefore, in practice Within the range 5 minutes to an indeterminate maximum time but it will depend upon the temperature.

It is believed, without prejudice to the invention, that the improvement in properties after heat-treatment is due to the carbide being fine and uniformly dispersed. At high temperatures (e.g. 1700" C.) the carbide is a coarse plate-like form and the ductility of the alloy is reduced. At low temperatures (e.g. 1200 C.) a fine carbide structure is obtained and the alloy possesses high ductility without sacrifice of strength. At intermediate temperatures a mixture of the two forms is obtained and such a structure is typical of alloys which have not been heat-treated. Such material is low in ductility both because of the presence of the coarse plate-like carbide and because the distribution of neither form of carbide is uniform. After the first stage of the heat-treatment at elevated temperatures a uniform dispersion of coarse plate-like carbide is obtained and this is transformed by the second heat-treatment into a uniform dispersion of fine carbide.

The upper limit of the temperature range in the first heat-treatment is not very precise but at temperatures above about 2000 C. the carbide is taken into solution causing embrittlement of the lattice and reprecipitation at grain boundaries on cooling. Below about 1600 C. the carbide is not uniformly distributed.

The upper limit of the temperature range in the second heat-treatment is set by the tendency for the coarse platelike carbide to be formed at elevated temperatures and by the tendency for the fine dispersion to coalesce. The lower limit is not very precise but it should be high enough for an approach to equilibrium to be obtained in reasonable time.

In alloys produced by some processes, for example, electron beam melting with a slow cooling rate, the carbide is predominantly of the coarse plate-like form in the ascast condition and because of the low ductility associated with this structure, subsequent processing is diflicult. A single heat-treatment in the range 1000 C. to 1350 C. will convert this carbide into a fine form and the material can then be processed into wrought forms, such as sheet.

When welding of the alloys takes place there are, in addition to the coarse structure of the fused metal, thermally disturbed zones in proximity to fused areas.

The properties of weld material can be improved in some cases by a single heat-treatment at a temperature in the range 1000 C.1350 C. A more uniform structure is obtained however by the two-stage heat-treatment -already described. Examples of the invention will now be particularly described.

Example I An alloy consisting of 11 Wt. percent tungsten, 3 wt. percent molybdenum, 2 wt. percent hafnium and 0.08 wt. percent carbon was arc-melted. The as-cast structure contained a precipitate of carbide in plate-like form and also some fine dispersion; these phases being formed during cooling of the ingot. The alloy was extruded at 1450 C. and subsequently forged and rolled to sheet 0.040 inch thick. The sheet was then recrystallised by heating to 1600 C. for 1 hour in vacuo, after which the carbide was in the form of uniformly dispersed coarse plates. This material was very notch sensitive at room temperature and a failure rate of 50% was experienced in bend tests at 4T. After a second heat-treatment at 1200 C. for 3 hours in vacuo the carbide was transformed into a fine uniformly dispersed structure. All material passed the 4T bend test and sheet could be hammered flat on itself without cracking.

Example II A second sample of the alloy was electron beam melted and cast and processed into sheet by a method similar to that described in Example I. The as-cast material con- Z3 tained areasonably uniformly dispersed carbide predominantly of coarse plate-like structure. The material was of low ductility and it was necessary to. pre-extrude it before forging and rolling. The sheet produced was of a limited ductility and the ductility was substantially increased by heating for 3 hours at 1200 C. in vacuo. A second sample of cast material was heat-treated for 3 hours at 1200 C. in vacuo before processing so as to transform the carbide into a fine uniformly dispersed structure. Ductility was increased and the material could be processed into sheet without pre-extrusion.

Example 111 Samples of the alloy sheet produced by the method described in Example I were argon arc-welded in air at a speed of 20 inches/minute. The weld material contained carbide which was predominantly of a coarse plate-like form and was of low ductility.

one sample ofthe weld material was heated for 3 hours at 1200 C. in vacuo and a substantial improvement in ductility was obtained, the material passing a bend test of 6T at room temperature. A second sample was heated in vacuo for 2 hours at 1700 C. followed by 3 hours at 1200 C. and a further increase in' ductility of the weld material was obtained.

We claim:

1. A method of heat-treating niobium-base alloys containing hafnium-bearing carbide comprising heating the said alloy to melt at least a portion thereof, slowly cooling said alloy to eifect solidification thereof and precipi Cir claimed in claim 1 in which the heating step which melts at least a portion of the alloy is a fusion welding process.

5. A method of processing an as-cast alloy consisting of 11% by weight of tungsten, 3% by weight of molybdenum, 2% by weight of hafnium and 0.08% by weight of carbon, balance niobium apart from impurities, said alloy being characterized by the presence of a precipitate of carbide in coarse plate-like condition, said method comprising heating the as-cast alloy at a temperature of 1200" C. for 3 hours in vacuo to transform the said coarse platelike precipitate into a fine uniformly dispersed carbide structure.

6. A method of heat-treating a welded article made from an alloy consisting of 11% by weight of tungsten, 3% by weight of molybdenum, 2% by weight of hafnium and 0.08% by weight of carbon balance niobium apart from impurities, said alloy being characterized by the presence of a precipitate of carbide in coarse plate-like condition, said method comprising heating the article after welding in vacuo at 1200 C. for 3 hours to transform the said coarse plate-like precipitate into a fine uniformly dispersed carbide structure.

7. A method of processing a niobium-base alloy containing a dispersion of hafnium-bearing carbide comprising melting the alloy in an electron beam, cooling the alloy slowly to effect solidification thereof and precipitation of the carbide in coarse plate-like condition, heating the alloy to a temperature in the range 1000 C.13S0" C. to transform the said coarse plate-like precipitate into a fine uniformly dispersed carbide structure, and subsequently processing the alloy into a wrought form.

References Cited UNITED STATES PATENTS 2,973,261 2/1961 Frank 75174 3,034,934 5/1962 Redden 14811.5 3,113,863 12/1963 Chang et al. 75l74 3,194,697 7/1965 Chang 148--133 3,230,119 1/1966 Gemmel et al. 148133 CHARLES N. LOVELL, Primary Examiner. 

